Vacuum arc devices utilizing symmetrical coaxial electrode structures



April 2s, 1970 J. A. RICH 3,509,406

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United States Patent 3,509,406 VACUUM ARC DEVICES UTILIZING SYMMET- RI'CAL COAXIAL ELECTRODE STRUCTURES Joseph A. Rich, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed July 1, 1968, Ser. No. 741,482 Int. Cl. H01j I7/04, 17/30, 21/22 U.S. Cl. 313-155 11 Claims ABSTRACT F THE DISCLOSURE Vacuum arc devices such as triggerable vacuum gap devices and vacuum switches are fabricated so as to include symmetrical coaxial structures which provide relatively large surfaces available to serve as the foot points of arc current conduction paths in relation to the volume of the arc device, while still retaining electrical symmetry between anode and cathode for alternating current operation. Added features of the invention include the possible incorporation of integral structures for providing rotational magnetic forces upon arc conduction paths.

This invention is related to my copending application, Ser. No. 639,693, filed May 19, 1967, and my concurrently-filed applications, Ser. Nos. 741,480 and 741,481, all of which are assigned to the present assignee, and incorporated herein by reference thereto.

In the fabrication of vacuum arc devices, such as triggerable vacuum gap devices and vacuum switches, the conventional prior art devices utilize electrode structures wherein the primary inter-electrode gap is substantially a plane-parallel gap and the arc-electrodes are substantially planar or of modified planar construction. Because of this the conventional prior art arc devices have a limited area of the arc electrodes thereof which are exposed to, and constitute the possible positions for the residence for arc conduction paths during arcing. Because of this, the current density at any given portion of such arcelectrodes tends to be relatively high, and the possibility of the formation of destructive anode spots is great. Additionally, since such structures result in essentially a longitudinal or axial path of current therethrough, the magnetic iields generated -by such current paths, on interaction with the arc conductions from electrode to electrode tends to cause a force which concentrates or bunches the conduction paths of the arc at the edges of the plane parallel electrodes resulting in the beginning of the formation of destructive anode spots at relatively low currents.

In my copending application, Ser. No. 639,693, mentioned hereinbefore, I have devised reentrant structures for arc-electrodes for vacuum arc devices which rely upon a minimization of the forces due to the interaction of current paths and magnetic fields to allow for a greater effective area for the arc-electrodes of the primary gap. While the devices in accord with the aforementioned application are exceedingly useful, particularly when it is desired to operate at exceedingly high Values of current which results in unacceptably high current densities in conventional plane parallel gap-type devices, the arcelectrodes of such devices are not electrically symmetrical. Accordingly, although the current carrying characteristic of these devices is excellent for both polarities of an alternating current voltage, the current carrying capacity before anode spot formation tends to be lesser when the smaller or inner arc-electrode constitutes the anode and, consequently, the formation of anode spots may occur at lower current values than is desirable.

Accordingly, an object of the present invention is to Patented Apr. 28, 1970 provide vacuum arc devices having arc-electrode structures which provide for a high effective arc-electrode surface while, at the same time, maintaining electrical symmetry between arc-cathode and arc-anode,

Still another object of the present invention is to provide vacuum arc devices wherein novel arc-electrode structures are utilized which structures may be physically placed within the body of vacuum arc devices presently manufactured on a commercial basis,

Yet another object of the present invention is to provide vacuum arc devices having -arc-electrodes with a reentrant coaxial structure for optimization of space requirements and minimization of current density at the arc-electrode surfaces,

Yet another object of the present invention is to provide devices of the type described having integral means for providing rotational forces to rotate arc conduction paths around an annular arcing surface to further minimize formation of destructive anode spots.

In accord with one aspect of the present invention, I provide vacuum arc devices having an evacuable envelope evacuated to a vacuum of 10*5 torr, or less, and including an insulating sidewall member separating, and hermetically sealed to, a pair of metallic endwall members. A pair of arc-electrodes that are substanti-ally identical are disposed within the envelope of the device and are so juxtaposed as to dene therebetween a primary breakdown gap. Each arc-electrode is comprised of a reentrant, concentric, cylindrical structure having a central member thereof connected to a metallic endwall member. Means are provided for supplying a concentration of electrical carriers to provide for breakdown of the primary arc gap :and electrical conduction thereby.

The novel features of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following detailed description, taken in connection with the appended drawings in which,

FIGURE 1 is a vertical cross-sectional view of a triggerable vacuum gap device constructed in accord with the present invention and utilizing a single trigger assembly,

FIGURE 2 illustrates, in vertical section, an alternative device to the embodiment of FIGURE 1 and illustrates a device having a plurality of trigger assemblies,

FIGURE 3 illustrates, in vertical section, a triggerable vacuum switch constructed in accord with the present invention and including both a trigger assembly and means for moving the arc-electrodes from a circuit-making to a circuit-breaking position,

FIGURE 4 is a`ve1tical cross-sectional view of an alternative embodiment of the invention wherein means are provided within each primary arc-electrode assembly for providing bucking magnetic fields within the primary arc gap to cause a rotation of arc conduction paths,

FIGURE 4(a), diagrammatic illustration of the magnet field existing within the device of FIGURE 4 during operation,

FIGURE 5 is a similar view to that of FIGURE 4 and illustrates a structure for providing integral means to cause additive magnetic fields within the primary arc gap for rotational purposes, and

FIGURE S(a), diagrammatic illustration of the magnetic field existing within the device of FIGURE 5 during operation,

FIGURE 6 is a vertical cross-sectional view of an alternative embodiment illustrating improved magnetic field-producing means incorporated within the arc-electrodes of the device.

In FIGURE 1 a triggerable vacuum gap, represented generally as 10, is illustrated in vertical cross-sectional View and includes an envelope 11 which comprises an insulating sidewall member 12 hermetically sealed with, physically joining, and electrically isolating an upper metallic endwall member 13 and a lower metallic endwall member 14. A first arc-electrode assembly 15 is suspended from, and electrically connected with, endwall member 13 and a second arc-electrode assembly 16 is physically mounted upon, and electrically connected with, lower endwall member 14. Arc-electrode assemblies 1'5 and 16 are juxtaposed with one another so as to form an annular primary arcing gap 17. A metallic shield member 18 having a generally cylindrical configuration, having generally inwardly tapered end portions and ferruled edges to prevent arcing, is juxtaposed within envelope 11 and Shields insulating sidewall member 12 from primary arcing gap 17, to preclude the deposition of metallic species from the arc thereupon to prevent short circuiting between endwall members 13 and 14.

A trigger assembly 19 is located adjacent an aperture in endwall member 14 and is adapted to cause the injection of an ionized electron-ion plasma through the hollow central region of arc-electrode 16 and into the region of primary arcing gap 17 to to cause the initia tion of a high current discharge between arc-electrode assemblies 15 and 16.

Arc-electrode 16 is comprised of an inner cylinder 20, having a first diameter which is large as compared with the arcing gap 17, which is mechanically and electrically afiixed to endwall member 14, for example, by Welding, brazing, or by threading with screw threads into mating threads in endwall member 14. Assembly 16 also includes an outer cylinder 21, concentric with cylinder 20, having a second diameter which is substantially greater than that of cylinder 20 and may conveniently be approximately twice the diameter thereof, Cylinder 21 is not connected at its outwardly-depending end to endwall member 14, but rather, is supported by connection of the inwardly depending end thereof to the inwardly-depending end of cylinder 20 by bridging member 22, which is substantially thicker in cross section than either cylinder 20 or 21. Bridging member 22 has a substantially annular configuration and is substantially planar at its inwardly-depending surface but is rounded at the edges thereof to preclude conduction paths which are seated thereupon from selectively becoming anchored to a sharp corner.

Similarly, arc electrode 15 comprises an inner cylinder 23-connected to endwall member 13, an outer cylinder 24 not directly connected to endwall member 13, and a bridging member 25 which connects the inwardly-depending ends of cylinders 23 and 24 and has an arcing surface 27 which, together with arcing surface 26 of arcelectrode 16 are ideally suited to form the main arcing surfaces of the device.

Arc-electrode assemblies 15 and 16 are formed of a high-purity metal that is free of gases or gas-forming compounds to a degree of at least no greater than one part per million of gas or gas-forming material contained therein. Such arc-electrodes may, for example, be composed of copper, beryllium, copper-beryllium alloys, copper-beryllium-bismuth alloys, or any of the materials set forth in the following patents: 2,975,256-Lee et al., 2,975,255-Laierty, 3,016,436-Lafferty, 3,140,373- Horn, and 3,246,979Laerty et al., for example. Preferably, these materials are prepared in high purified form, as for example, by multiple pass Zone refining, or by a special zone refining technique such as is set forth in Patent No. 3,234,351-Hebb. Insulating sidewall member 12 may be composed of Pyrex or a Nonex glass, or may be of a suitable ceramic, as f or example, high density alumina. Shield member 18 may conveniently be cornposed of stainless steel of other such relatively refractory materials which are easily heat treated to remove gaseous or gaseous-forming constituents by high ternperature bakeout in vacuo.

Trigger assembly 19 comprises a scored metallic or hydride film, for example, on a ceramic cylinder 29 with one side of the hydride or metal film connected-to metallic endwall member 14 and comprising a trigger cathode therefor, while the other, or inner side of the film, is connected to an inner trigger anode 28 comprising a cylindrical ceramic post, which has coated thereon a thin film of metal of hydride, which is connected by a wire running concentrically therethrough and emerging as trigger anode lead 30, This film is only coated upon anode post 28 above insulating washer 29 and the post constitutes an insulator therebelow. An insulating washer 31 supports ceramic washer 29 and the trigger aperture is closed and hermetically sealed, as for example, by the threading of an apertured metallic stopper 32, hermetically sealed to the-.anode post into a downwardly-depending flange member 33 which is integral with endwall member 14. An appropriate sealing gasket may, of course, be used therewith. Trigger device 19 may, for example, be a trigger assembly such as disclosed in Lafferty application, Ser. No. 564,132, filed July 1l, 1966, or may, alternatively, be that disclosed in Laiferty application, Ser. No. 704,935, filed Feb. 12, 1968, both of which are assigned to the present assignee.

In operation of the device of FIGURE 1, a high-voltage circuit, as for example one connected to an electric device to be protected from transient surges, is connected to arc-electrodes 15 and 16 by connecting to terminals 34 and 35 which are electrically connected to threaded terminal members 36 and 37. A pulse source 39 is conveniently connected to trigger anode lead 30. Alternatively, trigger anode 28 may be connected, through a suitable voltage-dropping resistance, to one side of the line. When a transient over-voltage occurs and it is desired to render the device 10 conductive, a pulse, sufficiently positive to cause the breakdown of the trigger gap across the scored face of the washer 29, is applied to trigger lead 30 causing a trigger arc to be established between the trigger cathode and the trigger anode. Heating of the metallic or hydride lm causes the thermal evolution of a charged electron-ion plasma therefrom which is propagated by the magnetic forces of the trigger arc upwardly through the hollow central portion of inner cylinder 20 of arc-electrode 16 and into inter-electrode gap 17, causing the establishment of a high-current electric arc therein.

If trigger electrode assemblies 15 and 16 were conventional abutting cylindrical electrodes or slight modifications thereof, the only portions thereof which would be available for the footpoints of conduction paths therebetween would be faces thereof facing one another, namely, the substantially plane-parallel faces. Under such circumstances, the current path through the device would essentially be longitudinal and an azimuthal magnetic field would be established thereabout which would force any other conduction paths downwardly toward the planeparallel arc-electrodes. The resultant configuration of the arc would then be a constricted conduction path or paths essentially at the outer corners of the plane-parallel arcelectrodes. This would cause a high current density which would cause the formation of destructive anode spots at relatively low total currents.

In accord with the present invention, however, the initial current conduction paths established by the injection of the electron-ion plasma into the interelectrode gap causes current paths to be established, not only between the adjacent surfaces 26 and 27 of arc-electrode assemblies 15 and 16, but also along the outer surfaces of outer cylinder 21 and the inner surface of inner cylinder 20. In practice, I find that arc-currents are established over the entire outer surface of outer cylinders 24 and 21 and the portion of the inner surfaces of inner cylinders 20 and 23 to a distance inwardly within the cylinder that is substantially equal to the radius of the inner cylinder.

In further accord with the concept of the electrode structure of the present invention, the current paths are not substantially longitudinal, since the portions of the total conduction current due to arc-current paths that terminate on the outer portions of outer cylinders 21 and 24 must pass inwardly toward the bridging members 22 and 25, thence across the bridging members and outwardly through inner cylinders 20 and 23. As used herein, the terms inwardly and outwardly have reference to direction with respect to the center of the device. Thus, the azimuthal magnetic fields which tend to be established by the electric currents in inner cylinders 20 and 23 are partially compensated by oppositely-directed azimuthal magnetic fields due to the currents within outer cylinders 21 and 24. This compensation is relatively slight at the outwardly-depending ends of cylinders 21 and 24. The net result is that, for all practical purposes, the arcing paths are forced slightly inwardly along the surface of cylinders of 21 and 24, 'but to only a degree such that greater than approximately 50 percent of the outer surfaces of cylinders 21 and 24, is available as arcing surfaces in a steady-state condition. This has been determined by observation of cylinder members of demountable systems which are constructed in accord with the present invention after arcing, since even moderate arcing causes a visible clouding of the surface polish of the arc-electrodes.

Although the dimensions of the components of the arc-electrode assemblies of devices in accord with the present invention may vary in accord with the currentcarrying capacity thereof, and consequently, the size of the device incorporating such assemblies, the following criterias are illustrative. Ideally, each arc-electrode assembly should be essentially as deep as it is wide. The diameter of the inner cylinder should be sufficient as to allow a substantial penetration of arc-conduction paths to the inner cylinder inner wall. Additionally, the difference between the diameter of the outer cylinder and the inner cylinder (the radial width of the bridging member therebetween) should be suiciently large as to define a relatively-broad, substantially-planar, primary gap with its counterpart on the other arc-electrode assembly. If this bridging member is too narrow the arc conduction paths tend to concentrate and bunch at a relatively small portion thereof, with resultant high current density. Ascribing to the inner radius of the inner cylinder the symbol (r); the outer radius of the outer cylinder (R); and the width of the bridging member (R-r), the following relationships should be met for optimum operation;

FIGURE 2 is a vertical cross-sectional view of a triggerable vacuum gap device, similar to that of FIGURE 1 of the drawing, wherein like numerals are utilized to identify like parts. The device of FIGURE 2 is identical to the device of FIGURE l, with the exception that, in addition to lower trigger assembly 19 juxtaposed adjacent the inner cylinder 20 of arc-electrode 16, a second upper trigger electrode assembly 19 is juxtaposed adjacent the inner cylinder 23 of arc-electrode 15. This permits a symmetrical operation of the device so that operation on alternating currents may be facilitated without any question of a delayed breakdown or failure to break down the primary arc gap, should the trigger be pulsed when the arc-electrode to which it is adjacent is not connected as arc-cathode. It is preferable that the trigger pulse be initiated with the trigger assembly adjacent the arc-electrode assembly which is to be the arc-cathode during the half cycle that the trigger assembly is activated. Accordingly, the embodiment of FIGURE '2 is specifically adapted to achieve this objective and the device is completely symmetrical to operate as a triggerable vacuum gap device in alternating current circuits.

FIGURE 3 illustrates, in vertical cross section, a triggerable vacuum switch constructed in accord with the present invention, similar to the devices of FIGURES 1 and 2, and utilizing like reference numerals for like parts. In FIGURE 3, envelope 11 includes an insulating sidewall member 12 and a pair of oppositely-disposed endwall members 13 and 14. As in the devices of FIG- URES 1 and 2, a pair of arc-electrode assemblies 15 and 16 are juxtaposed within envelope 11 so as to define an arcing gap 17. Similarly, a trigger electrode assembly 19 is juxtaposed adjacent the inner cylinder of arc-electrode 16. Unlike the embodiments of FIGURES l and 2, arcelectrode assembly 15 is not permanently aiiixed to endwall member 13. Rather, a base plate 50 encloses the outwardly-depending end of cylinder 23 and is affixed to an actuating rod 51 which passes from the evacuable envelope through flexible lbellows 52. Flexible bellows 52 allows a sufficient degree of longitudinal freedom so that actuating rod 51 may be moved longitudinally, as indicated by arrow 53, to cause the device to move between a circuit-making position in which surfaces 26 and 27 are abutted against one another, and a circuit-breaking position in which the surfaces 26 and 27 are withdrawn from one another.

In one embodiment of the present invention, the device may be constructed as illustrated and the trigger may be actuated just prior to the closing and contacting of the surfaces 26 and 27 with one another so as to stabilize and fix the time at which the device is changed from a circuit open to a circuit closed condition. Alternatively, trigger assembly 19 may be omitted and the device normally operated in a circuit-closed condition. In this instance, the separation of surfaces 26 and 27 may be relied upon as constituting the means for supplying an ionized electron-ion plasma within the device, since the mere act of withdrawal of surfaces 26 and 27 from one another causes an arc to be struck therebetween which immediately boils material from the arcing surfaces, which material is immediately ionized and constitutes the conducting medium of the vacuum switch device. In accord with both embodiments utilizing flexible bellows 52, the device in generally connected in circuit with an electric circuit or device, preferably in series therewith and may be doneI so by connecting to contact S4, which is connected to actuating rod 51 at 55, and to contact 56, which is connected with terminal member 57. For capacitor protection, on the other hand, the device is generally in parallel circuit with the capacitor bank.

FIGURE 4 of the, drawing illustrates an alternative l embodiment of the invention to that of FIGURE l, which may be incorporated in any of the other embodiments of the invention, and involves the incorporation of means for the provision of a magnetic field to provide a rotational force to cause the rotation of arc-current paths between arc-electrode assemblies 15 and 16 around the annulus, so as to further minimize the possibility of the formation of destructive anode spots. In FIGURE 4, the same reference numerals as utilized in FIGURES l, 2, and 3 are utilized to refer to like parts. 'In FIGURE 4, arc-electrode assemblies 15 and 16 are modied by causing inner cylinder 20 to be slotted with slots 60 and inner cylinder 23 to be slotted with Islots y61. Slots 60 and 61 are cut so as to define helical current paths between the respective endwall members 13 and 14, and the bridging members 25 and 22 of arc-electrodes 15 and 16, respectively. The multi-filar helices so formed are integral parts of cylinders 20 and 23. I have found, despite this, that the cylinders nevertheless maintain structural rigidity because the helical slots are not permitted to extend more than approximately 50% of the circumference of the cylinder in which the slots are cut, or about the circumference. Thus, any given slot 60 or 61 in traversing its path from the outwardly-depending end of cylinder 20 or 23 does not traverse more than approximately 180 of the periphery of the cylinder. If a greater amount of traverse were permitted for any given slot, in progressing from the -most outwardly-depending portion thereof to the most inwardly-depending portion thereof, a resultant mechanical instability may result and the electrical and mechanical forces due to the high current arcing tend to alter the physical position of the arc-electrode assemblies.

I am aware that helicoidal structures in arc-electrode devices have been discussed in the prior art, as for example, in U.S. Patent 3,014,107-Cobine et al., issued Dec. 19, 1961. In such structures, however, one must either tolerate mechanical instability and lack of rigidity or this lack must be compensated for by the addition of an additional arc-electrode support member.

In accord with the present invention, however, the requirement that no helical slot traverse more than approximately 180, makes it possible that current paths within cylinders 20 and 23 are rotated sufficiently as to cause a solenoidal type magneitc field to result therefrom without in any way compromising the rigidity of the arc-electrode structures.

In the device of FIGURE 4, the solenoidal slots 6-0 and 61 are cut so as to provide bucking magnetic fields which cause a cupsed field configuration within gap 17, illustrated diagrammatically in FIGURE 4a. This cupsed configuration causes a radial component of magnetic field Iwhich must be traversed by any arc-current conduction path between arc-electrodes 15 and 16. The interaction of the radial magnetic field and the conduction current within the conduction paths causes an orthogonal force to act thereupon, which force causes a rotation of the conduction paths azimuthally about the annulus off the arc-gap. This action is effective both upon current conduction paths that are passing directly between arc faces 26 and 27 and those which are passing between the outer surfaces of outer cylinders 21 and 24 and the inner surfaces of inner cylinders 20 and 23 Since the depth of penetration of the conduction paths into the inner cylinders 20 and 23 is to a depth of approximately the radius of the cylinder, and since it is desirable, if not necessary, that the helical slots be removed from any area at which the arc footpoints may dwell to avoid an anchoring effect at the sharp edges of the slot, the helical slots 60 and 61 are cut so as to terminate at a point which is no closer to the surfaces 26 and 27 than'approximately the radius of the inner cylinders 20 and 23.

FIGURE 5 of the drawing illustrates an alternative embodiment to the device of FIGURE 4 wherein like parts are identified with like reference numerals. In FIG- URE 5 the structure is identical to that of FIGURE 4 except that the slots 60 and 61 in cylinders 20 and 23, respectively, are cut so as to provide a reinforcing magnetic field rather than the bucking magnetic field in the vicinity of the gap 17 as in the device of FIGURE 4. The reinforcement of the magnetic field which is outwardly extensive in the region of gap 17 and has a radial component by virtue of this extension, which radial component serves the same purpose as the cupsed magnetic fields radial component in the device of FIGURE 4, so that rotation of the arcing paths around the annular gap and the attendant lowering of the probability of destructive anode spots results.

As in the device of FIGURE 4, the slots 60 and 61 do not approach the inwardly-depending end of the cylinders 20 and 23 any closer than the radius thereof so as to preclude the formation of an anchored spot upon the sharp edge of a helical slot. The magnetic eld utilized in this embodiment is illustrated diagrammatically in FIGURE 5a.

FIGURE 6` of the drawing illustrates yet another embodiment of the invention wherein the electrode assemblies 15 and 16 are further modified for incorporation of a multifilar helix in, and as an integral part of, each arc-electrode assembly and elimination of maintaining the arc-electrode helical slots at a distance from the inwardly-depending end thereof equal to the radius of the inner cylinder. As in the other figures, like elements in 8 FIGURE 6 to those in FIGURES 1-5 are identified with like reference numerals.

In FIGURE 6, arc-electrode assemblies 15 and 16 are disposed respectively in mechanical and electrical contact with endwall members 13 and 1-4. Similarly, arcelectrode assemblies 15 and 16 include support cylinders 20 and 23, respectively. Additionally, in accord with this embodiment of the invention, electrode assemblies 15 and 16 also include concentric inner unsupported cylinders `65'in arc-electrode assembly 15 and 66 in arc-electrode assembly 16. In each arc-electrode assembly, the inner concentric cylinders 65 and 66, respectively, like outer cylinders 21 and 24, but, rather, are joined at the inwardly-depending ends thereof with support cylinders 20 and 23 by means of bridging members 22 and 25, respectively.

In accord with this embodiment of the present invention, the inner cylinders 65 and 66 have substantially the same diameter as did the support cylinders of the devices of FIGURES 1 5 to facilitate entrance of conduction paths between arc-electrode assemblies 15 and 16 into the inner portion of the inner cylinders so as to increase the effective area of the arc-electrode assemblies. On the other hand, the outer support cylinders 21 and 24 are not of greater diameter. Outer cylinders 21 and 24 are of the same diameter as in the embodiments of FIGURES 1-5, and support cylinders 20 and 23 may be equidistant from inner and outer cylinders.

Also in accord with this embodiment of the present invention, since the portion of the arc-conduction paths which penetrate into the hollow cylindrical assemblies contacts inner cylinders 65 and 66 and since the helical slots 60 and 61 are made in support cylinders 20 and 23, respectively, no limitation need be placed upon the closeness to the arc gap which may be penetrated by the helical slots. Thus, the helical slots 60 and 61 are continued in cylinders 20 and 23, respectively, and terminate just short of bridging members 22 and 25. This makes possible the greatly increased radial magnetic field strength at the arc gap 17 and greatly inhances the rotatability of the arc conduction paths, without in any way, making possible the anchoring of arc conduction paths upon the sharp edge of the helical slot or the formation of the destructive anode spot thereat.

In further accord with this embodiment of the present invention, the helical slots may be in the same direction, so as to form a magnetic field having an outwardlyfringing bulbous shape in the vicinity of the arc gap, or may be in opposite directions so as to cause bucking magnetic fields and a cupsed magnetic field configuration in the arc gap, both of which are effective to cause the desired arc rotation.

In all of the embodiments of the present invention, it is to be noted that I have provided compact, relatively small arc-electrode structures which are compatable with and may be inserted within a vacuum switch or triggerable vacuum gap exterior envelope, such as disclosed in Patents 3,093,767-Lafferty and 3,246,979-Laierty et al., without requiring any change in the exterior contiguration of the envelope. On the other hand, by utilizing the unique electrode configuration of the present invention, I am able greatly to increase the effective area of the arc-electrodes so as greatly to decrease the current density of any portion of the arc-electrode assemblies and thereby greatly reduce the probability of the formation of destructive anode spots. By producing the folded cylindrical structures in a symmetrical array, I have provided devices which are equally suitable for triggering or for operation upon either cycle of an alternating current voltage and have provided devices which may be simultaneously triggered with pulses injected through either or both electrodes and have provided means for making one electrode movable to cause the devices to move from circuit-making to circuit-breaking positions and have further provide such means in connection with a trigger electrode assembly for producing such operation simultaneously with a predetermined and Well-controlled voltage breakdown.

While the structure as defined hereinbefore may be utilized in devices of varying sizes for varying current ratings, the following is a detailed description of such devices which have been constructed for 22,000 amperes operation and as illustrated in FIGURES 4 and 5. The sidewall members are a cylinder having a 3/a inch thick Pyrex glass wall thickness and an inside diameter of 6 inches. The inner dimension from endwall to endwall is 8 inches. The support cylinders 20 and 23 have a wall thickness of 143 inch, an inner diameter of 1% inches, an outer diameter of 11/2 inches, and a length of 2% inches. The outer cylinder has an outer diameter of 3 inches and a length of 21/2 inches. The helices are 180 slots, 1A; inch wide and 1/8 inch apart. A total of 8 slots are cut in the inner cylinders. When such devices were connected across a voltage of 50 kv. and a trigger assembly having a hydride (TiH4) film was pulsed with 500 volts for 100 microseconds, a current of 22,000 amperes was sustained for the remainder of the half cycle, with rotation, and without visible evidence of melting of either arc-electrode assembly. Arc tracks showed that the arc-current conduction paths had occupied approximately 1/2 the surface of the outer cylinders, all of the bridging members, and approximately S; inch into the inner cylinder.

While the invention has been set forth herein with respect to certain examples and embodiments thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended claims, I intend to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum arc device comprising:

(a) an hermetically sealed envelope including a pair of conductive endwall members and an insulating sidewall member sealed to and maintaining electrical isolation therebetween (al) said envelope being evacuated to a pressure of -5 torr or less (b) a pair of substantially identical arc-electrodes one each of which is connected to a respective one of said endwall members and juxtaposed with one another within said envelope so as to define an annular primary arc gap,

(b1) each of said arc-electrodes being in the form of a reentrant cylindrical structure including an inner support cylinder having a first diameter aixed to one endwall member, an outer concentric cylinder having a second diameter and not contacting said endwall member, and a bridging member connecting the inwardly-depending ends of said cylinders and constituting an arc-supporting member for said arc-electrode (c) a shield member surrounding said primary arc gap (d) means within said envelope for providing an electron-ion plasma within said envelope to initiate an electric arc between said arc-electrodes, and

(e) means for connecting said device in circuit with an electric line to be controlled, switched, or protected.

2. The devices of claim 1 wherein said arc-electrode assemblies are each substantially as deep as they are wide.

3. The device of claim 1 wherein the diameter of said inner cylinder of said arc-electrode assembly is greater than one half the diameter of said outer cylinder thereof.

4. The device of claim 1 wherein said means for providing an electron-ion plasma within said envelope is a trigger arc assembly.

5. The device of claim 4 wherein a trigger arc assembly is juxtaposed adjacent each of said arc-electrode assemblies.

6. The device of claim 1 wherein the means for producing an electron-ion plasma within said envelope is means to move one of said arc-electrode assemblies from a circuit-making to a circuit-breaking position.

7. The device of claim 6 wherein the other of said arcelectrode assemblies has a trigger arc assembly juxtaposed adjacent thereto.

8. The device of claim 1 wherein said inner cylinders of said arc-electrode assemblies are slotted so as to form a multifilar helix therein, said slots being cut at such an angle as to traverse no greater than approximately of the circumference of such cylinders.

9. The device of claim 8 wherein said helices are each in the same direction so as to induce additive magnetic fields in the vicinity of said primary arc-gaps.

10. The device of claimvS wherein said helices are in opposite directions so as to induce oppositely poled magnetic fields within said primary arc-gap.

11. The device of claim 8 `wherein an additional inner cylinder is afixed to each of said helix-containing cylinders at the inwardly-depending end thereof to isolate said helix from arc conduction paths to said arc-electrode assembly during arcing.

References Cited UNITED STATES PATENTS 3,246,979 4/1966 Lafferty et al. 200-144 X 3,320,478 5/1967 Harrison 313-155 X 3,328,632 6/1967 Robinson 313-233 X 3,331,981 7/1967 Lafferty 313-233 X 3,417,216 12/1968 Smith 200-144 3,450,922 6/1969 Gallagher 313-155 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. C1. X.R. 313-198, 217, 233 

